Gas separation is useful in many industries and can typically be accomplished by flowing a mixture of gases over an adsorbent material that preferentially adsorbs one or more gas components, while not adsorbing one or more other gas components. The non-adsorbed components are recovered as a separate product.
One particular type of gas separation technology is swing adsorption, such as temperature swing adsorption (TSA), pressure swing adsorption (PSA), partial pressure purge swing adsorption (PPSA), rapid cycle pressure swing adsorption (RCPSA), rapid cycle partial pressure swing adsorption (RCPPSA), and not limited to but also combinations of the fore mentioned processes, such as pressure and temperature swing adsorption. As an example, PSA processes rely on the phenomenon of gases being more readily adsorbed within the pore structure or free volume of an adsorbent material when the gas is under pressure. That is, the higher the gas pressure, the greater the amount of readily-adsorbed gas adsorbed. When the pressure is reduced, the adsorbed component is released, or desorbed from the adsorbent material.
The swing adsorption processes (e.g., PSA and TSA) may be used to separate gases of a gas mixture because different gases tend to fill the micropore of the adsorbent material to different extents. For example, if a gas mixture, such as natural gas, is passed under pressure through a vessel containing an adsorbent material that is more selective towards carbon dioxide than it is for methane, at least a portion of the carbon dioxide is selectively adsorbed by the adsorbent material, and the gas exiting the vessel is enriched in methane. When the adsorbent material reaches the end of its capacity to adsorb carbon dioxide, it is regenerated in a PSA process, for example, by reducing the pressure, thereby releasing the adsorbed carbon dioxide. The adsorbent material is then typically purged and repressurized. Then, the adsorbent material is ready for another adsorption cycle.
The swing adsorption processes typically involve one or more adsorbent bed units, which include adsorbent beds disposed within a housing configured to maintain fluids at various pressures for different steps in an adsorption cycle within the unit. These adsorbent bed units utilize different packing material in the bed structures. For example, the adsorbent bed units utilize checker brick, pebble beds or other available packing. As an enhancement, some adsorbent bed units may utilize engineered packing within the bed structure. The engineered packing may include a material provided in a specific configuration, such as a honeycomb, ceramic forms or the like.
Further, various adsorbent bed units may be coupled together with conduits and valves to manage the flow of fluids. Orchestrating these adsorbent bed units involves coordinating the cycles for each adsorbent bed unit with other adsorbent bed units in the system. A complete PSA cycle can vary from seconds to minutes as it transfers a plurality of gaseous streams through one or more of the adsorbent bed units.
While conventional glycol absorption processes for dehydration of feeds, such as natural gas, are established and low cost processes, glycol absorption does not provide the level of dehydration required for certain recovery processes, such as cryogenic processing of natural gas, for example, to recover natural gas liquids (NGLs). For example, the water content of glycol dehydrated natural gas is relatively low (e.g., between 100 parts per million molar (ppm) and 200 ppm) at typical field dehydration specifications, but has to be reduced to less than 1 ppm, or even less than 0.1 ppm, for cryogenic processing.
Conventional dehydration of natural gas streams for subsequent cryogenic processing is accomplished using a TSA molecular sieve adsorption process. In the TSA molecular sieve adsorption process, the natural gas flows through molecular sieve adsorbent beds that extract the water from the gas in the stream. Several adsorbent beds are arranged in parallel to provide one or more molecular sieve adsorbent beds performing the adsorption step (e.g., adsorbing water from the stream), while one or more of the other molecular sieve adsorbent beds are performing regeneration steps (e.g., offline for regeneration to remove adsorbed contaminants from the adsorbent bed). When the molecular sieve adsorbent bed is almost saturated, the molecular sieve adsorbent bed is placed into a regeneration step (e.g., taken offline) and a portion of the dry gas product stream is heated to about 500° F. (260° C.) in a fired heater and directed through the molecular sieve adsorbent bed to raise the temperature and desorb the water from the molecular sieve adsorbent bed. The wet regeneration gas (e.g., gas with the desorbed water from the bed) is then cooled outside the bed to condense out the water and the gas is recycled into the feed stream upstream of the dehydration system. Unfortunately, for typical NGL recovery plants, such as a cryogenic NGL recovery plants, the molecular sieve adsorbent beds require large high pressure vessels and involve large volumes of gas and adsorbent material. As the TSA molecular sieve adsorption process operates at feed stream pressure, the units involve high pressures, contain a large inventory of adsorbent material, are heavy, have a large footprint, and are costly to operate. Also, the duration of the thermal swing cycle is two or more hours as the adsorption front progresses through the majority of the molecular sieve adsorbent bed's length. The TSA molecular sieve adsorption process also requires a regeneration gas fired heater that uses significant amounts of fuel and requires a large footprint due to the safety spacing requirements for fired elements.
Conventionally, following its regenerating of the wet adsorbent beds, the wet regeneration gas is recycled to the feed stream upstream of the dehydration system or used as process plant fuel. To avoid excessive recycle, the volume of the dry gas that can be used for regeneration is limited to a small percentage of the feed stream volume, typically less than ten percent. With a relatively low volume of regeneration gas and the need to nearly completely dehydrate the adsorbent bed during regeneration, a high regeneration temperature of about 500° F. (260° C.) or more is needed to completely regenerate the molecular sieve adsorbent beds during each cycle. Even when the regeneration gas is limited to 500° F. (260° C.), the temperature of the regeneration gas can eventually cause hydrothermal degradation of the adsorbent particles and coke formation within the bed leading to deactivation, which is further increased with higher temperatures of the purge stream. Additionally, the use of a fired heater in a natural gas plant requires increased equipment spacing for risk mitigation, which is particularly costly in an offshore facility.
As another approach, a PSA molecular sieve adsorption process may be used for the process. This approach uses a low flow stream of purge gas at a low pressure to regenerate the molecular sieve adsorbent beds. Unfortunately, this process includes recycle compression for typical natural gas dehydration applications. As obtaining high regeneration gas temperatures is less costly than recycle compression, the PSA molecular sieve adsorption process is more costly than the TSA molecular sieve adsorption process noted above.
Accordingly, there remains a need in the industry for apparatus, methods, and systems that provide enhancements to the processing of feed streams with adsorbent beds, which may be integrated with recovery equipment. The present techniques provide enhancements by utilizing PPSA processes to regenerate adsorbent beds at lower pressure and temperatures than those utilized in conventional molecular sieve TSA and PSA approaches. The present techniques overcomes the drawbacks of conventional molecular sieve TSA and PSA approaches by using larger purge gas volumes (e.g., ten to twenty times greater than in conventional molecular sieve TSA and PSA approaches). Further, a need remains for an approach that does not involve the use of purge gases heated to higher temperatures (e.g., at above 500° F. (260° C.)) or the use of fired heaters.